The world’s largest airlines collected an estimated $21.46 billion in passenger fees and other extra revenue last year, about double the amount collected in 2008, according to a new study.

For some airlines, revenue generated from extra fees, the sale of frequent flier points, and commissions for booking passengers into hotel rooms and rental cars now represents 15 percent to 30 percent of all airline revenue, according to the report by IdeaWorks Co., a Wisconsin consultant on airline fees, and Amadeus Corp., a Madrid technology company for the travel industry.

The annual report released Tuesday said 47 airlines worldwide reported some type of ancillary revenue last year, compared with only 23 airlines in 2007.

United Airlines and Continental Airlines, which have merged but operate as two airlines, reported a combined $5 billion in ancillary revenue in 2010, the most for any airline, followed by Delta Air Lines with $3.7 billion and American Airlines with $1.9 billion, according to the report.